Polyester resins are well known in the art, where an alkyd ingredient is reacted with about 50 wt.% of a vinyl monomer such as styrene, to provide a fast drying resin which may be diluted by addition of aromatic naptha or other solvent, as taught by Yetkon, in U.S. Pat. No. 3,743,615. The use of low molecular weight materials and large amounts of solvents however provided films which tend to be soft, and posed fire and ecology problems. Cummings, in U.S. Pat. No. 3,620,989, attempted to solve these problems by using a water emulsion, oil based copolymer of glycerol, soya oil, maleic anhydride, phthalic acid and ethylhexyl acrylate or vinyl acetate, with a large excess of maleic anhydride over phthalic acid. Sattler, in U.S. Pat. No. 4,077,925, U.S. Ser. No. 745,727, filed Nov. 26, 1976 provided a nonaqueous polyester resin, having a high temperature capability, for use in impregnating thick mica insulation. This resin comprised a fatty acid oil based alkyd component, reacted with an alkyl acrylate monomer. Such fatty acid oil based compositions, however, while having outstanding thermal properties, could not easily be coated onto metal substrates at thicknesses of under about 0.5 mils (0.0005 inches). While photoinitiators could be present to initiate gelation, U.V. curing provided soft films, and a complete cure required the application of heat.
The cores of transformers and generators consist of laminations stamped from a continuous roll of steel. It is preferred to have an ultra thin, very tough insulating layer between each lamination. The thinner the insulation coating, the less bulky the core. A great deal of time and labor would be saved if the steel roll could be continuously coated with a very thin layer of insulating material before the stamping operation. This would require a very low viscosity coating resin, capable of fast application and pinhole free cure, to form a tough film. Coating metals also presents special problems of metal wetting and film adhesion.
With regard to metal coating compositions, Hall, in U.S. Pat. No. 3,669,825, taught 10 mil thick, photosensitive, metal coating compositions. The compositions comprised a catalyst-free polyester mixed with a styrene vinyl monomer, in association with an organic substituted ester of a metal acid, like titanic or zirconic acid, as an adhesion-promoting agent. The coating composition was applied continuously, to a substrate on a conveyor moving at about 20 ft./min. The coating was curable in such thick layers, without catalysts, by initial radiation treatment, followed by a complete heat cure, or by use of high energy radiation, such as electron bombardment by radium or strontium 90.
Fitko, in U.S. Pat. No. 3,567,494, did not require oven baking cycles or use of dangerous radiation sources, providing thin, ultraviolet radiation curable, metal coating compositions that did not require adhesion-promoting agents to wet the metal. The coating compositions comprised acrylic esters, prepared by coesterification, under a nitrogen atmosphere, and with a condensation catalyst, of a polyhydric alcohol, with acrylic acid, a monofunctional aliphatic acid, and a saturated aliphatic or unsaturated dicarboxylic acid, in a toluene medium. After solvent removal, a viscous, acrylic ester reaction product was provided, which could be ultra-violet radiation cured, to form 0.1 to 2 mil thick films. Such films however would have poor thermal properties.
What is needed is a 100% solids, oil free polyester resin composition, having good pot life, low volatility, good thermal properties, a low viscosity without addition of non-reactive diluents or solvents, and which can be quickly coated on metallic foil or strips at thicknesses below about 5 mils, and preferably below about 0.5 mil. It should quickly cure, solely under ultraviolet radiation without heating, without surface inhibition, to a hard, scratch resistant, tough insulation, with no solvent removal, or fire hazard problems.